Pressurized containers such as compressed gas containers are normally equipped with temperature or pressure activated valves. Once the pressure within the container rises above a predetermined value, the pressure activated valve will open to allow the internal gas to escape. Such containers can also be subjected to extreme heat, either through the action of a fire or otherwise, and the pressure within the container can increase as the temperature increases. If the gas is not allowed to vent, the pressure can build until the stresses exerted on the inside walls of the container exceed the strength of the container itself. At this point, the cylinder could rupture explosively releasing the contents and potentially some shrapnel. In the case of a fire, it is possible that the pressurized gas stored within the container can be heated to an extreme temperature very quickly, causing the container to explode even though the pressure activated valve has opened. The reason for this is that the gas cannot escape fast enough to prevent an explosion. Also, for some partially filled aluminum cylinders, the walls of the cylinders can soften and rupture even when the pressure within the cylinder is below the relief setting on the pressure activated valve.
To avoid such catastrophic failures, various types of safety relief valves have been invented. U.S. Pat. Nos. 2,040,776; 3,472,427; and 4,059,125 teach three different types of relief valves having a fusible material which can be ejected under extreme temperature or pressure conditions. U.S. Pat. No. 4,352,365 teaches a safety valve which can be activated by either pressure or temperature. One drawback of these valves is that the fusible material tends to extrude out of the bore after a relatively short period of time. Such extrusion can cause leakage and premature failure of the valve itself. Other U.S. patents of interest include: 1,211,173; 1,303,248; 1,876,938; 1,984,375; 2,020,075; 3,554,227; 4,335,734; 4,407,432; and 4,506,423. However, some of these devices have proved costly to manufacture and some are ineffective at high temperatures over long time periods.
Now a thermally activated valve has been invented which will essentially prevent a fusible material from extruding out of a uniquely designed helical passageway until a selected temperature value has been reached or exceeded.